METHOD FOR FORMING A FILM OF  AN OXIDE OF In, Ga, AND Zn

ABSTRACT

A method for forming a film of an oxide of In, Ga, and Zn, having a spinel crystalline phase comprises providing a substrate in a chamber; providing a sputtering target in said chamber, the target comprising an oxide of In, Ga, and Zn, wherein: In, Ga, and Zn represent together at least 95 at % of the elements other than oxygen, In represents from 0.6 to 44 at % of In, Ga, and Zn, Ga represents from 22 to 66 at % of In, Ga, and Zn, and Zn represents from 20 to 46 at % of In, Ga, and Zn; and forming a film on the substrate, the substrate being at a temperature of from 125° C. to 250° C., by sputtering the target with a sputtering gas comprising O2, the sputtering being performed at a sputtering power of at least 200 W.

INCORPORATION BY REFERENCE TO RELATED APPLICATION

Any and all priority claims identified in the Application Data Sheet, orany correction thereto, are hereby incorporated by reference under 37CFR 1.57. This application claims the benefit of European PatentApplication No. EP 19212086.3, filed Nov. 28, 2019. The aforementionedapplication is incorporated by reference herein in its entirety, and ishereby expressly made a part of this specification.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of semiconducting materialsand more specifically to a film of an oxide of In, Ga, and Zn and to amethod for making the same.

BACKGROUND OF THE INVENTION

InGaZnO₄ (IGZO) is a conductive transparent oxide typically deposited byphysical vapor deposition (PVD). IGZO has a low off-state current(I_(off)) and moderate mobility. This makes it a good candidate forforming field effect devices in relatively low-temperatureFront-End-Of-Line (FEOL) and Back-End-Of-Line (BEOL) process sequences.Such a material allows the integration of electrical interconnects withelectrical switching on top of front-end-of-line (FEOL) devices. Also,the low off-current behavior (<1×10⁻²¹ A/μm) of this material opens thepossibility of its integration in various memory applications. Dependingon the conditions used for its formation, so far, IGZO has been obtainedin an amorphous phase, in a so-called C-axis aligned crystalline (CAAC)phase, or in mixtures thereof. The CAAC phase is characterized by havinga repeated structure when seen from the c-axis direction and has orderedInO planes with distances equal to those found in (poly-)crystallinehexagonal structures. The CAAC phase is amorphous when seen from thedirection perpendicular to the c-axis.

The CAAC phase is advantageous because its structure is close tohexagonal crystalline IGZO which has a reduced electron effective masswhen compared to the amorphous phase. This should in principle lead tohigher electron mobilities. If scattering mechanisms are taken intoaccount, the mobility will be lower. This is the reason why the mobilityobserved for amorphous IGZO films remains similar to the mobilityobserved for films comprising the CAAC phase.

Both phases are able to have relative moderate electron mobility in therange of 10-30 cm²/Vs.

US2019024227 discloses metal oxide films including In, M, and Zn whereinM is Al, Ga, Y, or Sn aiming at improving the electron mobility. In thisrespect, the best film disclosed in US2019024227 is sample A3 which hasan In:Ga:Zn atomic ratio of 4:2:4.1 and has been formed by deposition ona substrate at 130° C. with an argon flow rate of 90% and an oxygen flowrate of 10%.

There is however a need in the art for alternative materials having thepotential to further improve electron mobility.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide good films of anoxide of In, Ga, and Zn, having a spinel crystalline phase, devicescomprising the same, and methods for forming the same.

The above objective is accomplished by methods, a films, and devicesaccording to the present invention.

In a first aspect, the present invention relates to a method for forminga film of an oxide of In, Ga, and Zn, having a spinel crystalline phase,the method comprising:

a. Providing a substrate in a chamber,

b. Providing a sputtering target in said chamber, the target comprisingan oxide of In, Ga, and Zn, wherein:

-   -   In, Ga, and Zn represent together at least 95 at % of the        elements other than oxygen,    -   In represents from 0.6 to 44 at % of In, Ga, and Zn,    -   Ga represents from 22% to 66 at % of In, Ga, and Zn, and    -   Zn represents from 20 to 46 at % of In, Ga, and Zn,

c. Forming a film on the substrate, the substrate being at a temperatureof from 125° C. to 250° C., by sputtering the target with a sputteringgas comprising O₂, the sputtering being performed at a sputtering powerof at least 200 W.

Theoretical calculations have led the present inventors to realize thatoxides of In, Ga, and Zn in a spinel crystalline phase have a lowereffective electron mass than such oxides in an amorphous or CAAC phase.This results in a higher electron mobility for spinel oxides than forCAAC or amorphous oxides. Realizing this, the present inventors havefound a method for forming films of an oxide of In, Ga, and Zn, having aspinel crystalline phase. A higher electron mobility permits, forinstance, to form field-effect transistors with a higher drive current,which is very advantageous for meeting the high read speed specificationof direct random access memory devices.

It is an advantage of embodiments of the present invention that the filmformed, due to the presence of the spinel phase, may have more uniformelectron mobility than similar films having a CAAC phase instead of thespinel phase.

It is an advantage of embodiments of the present invention that thefilms obtained, due to the presence of the spinel phase, may be moreresistant to thermally-induced phase separation than similar filmswithout the spinel phase. This is especially true for thermally-inducedphase separation in the presence of hydrogen because the films accordingto embodiments of the present invention may be more resistant againsthydrogenation than films not comprising the spinel phase.

It is an advantage of embodiments of the present invention that thefilms obtained, due to the presence of the spinel phase, may be morestable under high voltage.

It is an advantage of embodiments of the present invention that a filmof an oxide of In, Ga, and Zn, having a spinel crystalline phase, can beobtained without annealing with an energy beam such as a laser.

This is advantageous because a more homogeneous film can be obtained.Also, an homogeneous film can typically be obtained faster than if alaser was used. Also, since no laser light source is needed, the methodcan be less expensive to implement.

The film can be of any thickness. For instance, it can be from 20 to 200nm thick. The film has a spinel phase. In embodiments, the film mayfurther have an amorphous phase. In embodiments, the film may furtherhave a CAAC phase. In embodiments, the film may have a spinel phase, anamorphous phase, and a CAAC phase.

In embodiments, the spinel crystalline phase may represent from 10 vol %to 100 vol % of the film. The rest of the film is typically an amorphousphase and a CAAC phase. Measuring the extent of the spinel crystallinephase in the film can be performed by Θ-2Θ (out-of-plane) XRD. Since theamorphous phase only gives a weak signal by XRD, a direct evaluation ofthe extent of the spinel crystalline phase is not convenient. It ispreferred to use an indirect measurement method involving acrystallization of the amorphous phase before performing the XRDmeasurement. For this purpose, the amorphous phase may be transformedinto a crystalline phase (typically an hexagonal phase) by performing anoxygen anneal at 700° C. In order to be able to interpret quantitativelythe XRD results, a calibration can be performed. For this purpose, thesignal intensity corresponding to a 100 vol % amorphous sample (aftercrystallization by an oxygen anneal at 700° C.) and the signal intensitycorresponding to a 100 vol % CAAC sample are preferably measured.Finally, the Θ-2Θ (out-of-plane) XRD signals of the sampled of interest(after oxygen anneal) can be compared to the calibration signals and therelative vol % of spinel crystalline phase can be determined bysubtracting from 100 vol % the vol % corresponding to the crystallizedamorphous phase and to the CAAC phase.

In embodiments, the spinel crystalline phase may be of the space groupFd3m. The space group Fd3m is also known as the space group 227.

The presence of the spinel crystalline phase of the space group Fd3m canbe determined by the presence of peaks at Θ=17.4° and 35.5o inout-of-plane XRD (see FIG. 3) and at 17.4° and 60° in grazing incidenceXRD.

The presence of the CAAC in the film can be detected by out-of-planeXRD, when a broad peak in the range Θ=29° to 31° appears.

The oxide formed by the method typically comprises an oxide of In, Ga,and Zn, wherein:

-   -   In, Ga, and Zn represent together at least 95 at % of the        elements other than oxygen,    -   In represents from 0.6 to 44 at % of In, Ga, and Zn,    -   Ga represents from 22 to 66 at % of In, Ga, and Zn, and    -   Zn represents from 20 to 46 at % of In, Ga, and Zn,

Typically, the chemical composition of the oxide formed by the method isthe same as the composition of the target.

In embodiments, the oxide formed by the method may comprise an oxide ofIn, Ga, and Zn, wherein:

-   -   In represents from 2 to 40 at % of In, Ga, and Zn,    -   Ga represents from 25 to 55 at % of In, Ga, and Zn, and    -   Zn represents from 23 to 43 at % of In, Ga, and Zn.

In embodiments, the oxide formed by the method may comprise an oxide ofIn, Ga, and Zn, wherein:

-   -   In represents from 10 to 40 at % of In, Ga, and Zn,    -   Ga represents from 30 to 50 at % of In, Ga, and Zn, and    -   Zn represents from 27 to 40 at % of In, Ga, and Zn.

In embodiments, the oxide formed by the method may comprise an oxide ofIn, Ga, and Zn, wherein:

-   -   In represents from 10 to 35 at % of In, Ga, and Zn,    -   Ga represents from 30 to 50 at % of In, Ga, and Zn, and    -   Zn represents from 27 to 40 at % of In, Ga, and Zn.

In embodiments, the oxide formed by the method may comprise an oxide ofIn, Ga, and Zn, wherein:

-   -   In represents from 30 to 35 at % of In, Ga, and Zn,    -   Ga represents from 30 to 35 at % of In, Ga, and Zn, and    -   Zn represents from 30 to 35 at % of In, Ga, and Zn.

In embodiments, the oxide formed by the method may comprise an oxide ofIn, Ga, and Zn, wherein the In, Ga, and Zn represent together at least99 at %, preferably at least 99.9 at %, more preferably at least 99.95%of the elements other than oxygen.

In embodiments, the oxide formed by the method may consist of an oxideof In, Ga, and Zn, and usual impurities. Usual impurities may forinstance be Fe, Ni and Si (e.g. in trace amounts). However, impuritiesare typically not detectable.

In embodiments, the oxide formed by the method may consist of an oxideof In, Ga, and Zn.

In embodiments, the ratio [In] on [Ga] in the oxide formed by the methodmay be from 0.01 to 2, preferably from 0.1 to 1.98, more preferably from0.3 to 1.97, yet more preferably from 0.51 to 1.96, yet more preferablyfrom 0.7 to 1.5, even more preferably from 0.8 to 1.2, most preferablyfrom 0.9 to 1.1.

In embodiments, the ratio [In] on [Zn] in the oxide formed by the methodmay be from 0.02 to 2.2, preferably from 0.1 to 2.2, more preferablyfrom 0.5 to 2.2, yet more preferably from 0.7 to 2.2, yet morepreferably from 0.8 to 1.2, most preferably from 0.9 to 1.1.

In embodiments, the ratio [Ga] on [Zn] in the oxide formed by the methodmay be from 0.5 to 2.2, preferably from 0.7 to 2.2, more preferably from0.8 to 1.2, most preferably from 0.9 to 1.1.

The substrate can be made of any material. In embodiments, the substratemay comprise a semiconductor material (e.g. Si). For instance, thesubstrate may be a Si wafer such as a 300 mm diameter Si wafer. Inembodiments, the substrate may comprise a dielectric material (e.g.SiO₂). In embodiments, the substrate may comprise a semiconductormaterial and a dielectric material. For instance, it may comprise asemiconductor substrate on which an oxide layer is present, and thedeposition may be performed on the oxide layer (e.g. SiO₂ on Si). Thepresence of the oxide layer may be due to the deposition of such a layeron the semiconductor material or may be due to the oxidation of the topsurface of the semiconductor material. The presence of the oxide layeris advantageous when conductivity measurements need to be performed onthe oxide of In, Ga, and Zn.

In embodiments, step a may comprise heating up the substrate so as toremove moisture adsorbed thereon. For instance, the substrate may beheated at a temperature of from 100 to 450° C., preferably from 200 to425° C., yet more preferably from 300 to 400° C. (e.g. 350° C.) under aninert atmosphere. Ar is preferred for the inert atmosphere. The heatingup step can, for instance, last from 10 s to 5 min, preferably from 20 sto 3 min, more preferably from 30 s to 2 min, yet more preferably from45 s to 75 s (e.g. 1 min).

The chamber is typically a PVD chamber. Before step c, the chamber istypically under vacuum (e.g. from 0.5×10⁻⁵ to 9×10⁻⁵ (e.g. 3×10⁻⁵) Torr.Placing the substrate in that chamber can, for instance, be performed byrobotic handling. The chamber may comprise a chuck for electrostaticallyclamping and heating the substrate. The electrostatic clamping can, forinstance, be caused by a bias voltage of 300 V.

The chuck may comprise channels for allowing a back-side gas flow. Inembodiments, the substrate may be on a chuck through which at least 0.5sccm and preferably from 2 to 5 sccm of an inert gas (e.g. Ar) may flowto enable (proper) heat transfer.

Step b of providing the sputtering target in said chamber can beperformed before, after, or simultaneously with step a.

The target comprises an oxide of In, Ga, and Zn, wherein:

-   -   In, Ga, and Zn represent together at least 95 at % of the        elements other than oxygen,    -   In represents from 0.6 to 44 at % of In, Ga, and Zn,    -   Ga represents from 22 to 66 at % of In, Ga, and Zn, and    -   Zn represents from 20 to 46 at % of In, Ga, and Zn,

On the high side of the In range, better electron mobility can beachieved than on the lower side. On the lower side of the In range,smaller leakage currents can be achieved than on the higher side.

In embodiments, the sputtering target provided in step b may comprise anoxide of In, Ga, and Zn, wherein:

-   -   In represents from 2 to 40 at % of In, Ga, and Zn,    -   Ga represents from 25 to 55 at % of In, Ga, and Zn, and    -   Zn represents from 23 to 43 at % of In, Ga, and Zn.

In embodiments, the sputtering target provided in step b may comprise anoxide of In, Ga, and Zn, wherein:

-   -   In represents from 10 to 40 at % of In, Ga, and Zn,    -   Ga represents from 30 to 50 at % of In, Ga, and Zn, and    -   Zn represents from 27 to 40 at % of In, Ga, and Zn.

In embodiments, the sputtering target provided in step b may comprise anoxide of In, Ga, and Zn, wherein:

-   -   In represents from 10 to 35 at % of In, Ga, and Zn,    -   Ga represents from 30 to 50 at % of In, Ga, and Zn, and    -   Zn represents from 27 to 40 at % of In, Ga, and Zn.

In embodiments, the sputtering target provided in step b may comprise anoxide of In, Ga, and Zn, wherein:

-   -   In represents from 30 to 35 at % of In, Ga, and Zn,    -   Ga represents from 30 to 35 at % of In, Ga, and Zn, and    -   Zn represents from 30 to 35 at % of In, Ga, and Zn.

In embodiments, the sputtering target provided in step b may comprise anoxide of In, Ga, and Zn, wherein the In, Ga, and Zn represent togetherat least 99 at %, preferably at least 99.9 at %, more preferably atleast 99.95% of the elements other than oxygen.

In embodiments, the sputtering target provided in step b may consist ofan oxide of In, Ga, and Zn, and usual impurities. Usual impurities mayfor instance be Fe, Ni and Si (e.g. in trace amounts).

In embodiments, the sputtering target provided in step b may consist ofan oxide of In, Ga, and Zn.

In embodiments, the ratio [In] on [Ga] in the sputtering target may befrom 0.01 to 2, preferably from 0.1 to 1.98, more preferably from 0.3 to1.97, yet more preferably from 0.51 to 1.96, yet more preferably from0.7 to 1.5, even more preferably from 0.8 to 1.2, most preferably from0.9 to 1.1.

In embodiments, the ratio [In] on [Zn] in the sputtering target may befrom 0.02 to 2.2, preferably from 0.1 to 2.2, more preferably from 0.5to 2.2, yet more preferably from 0.7 to 2.2, yet more preferably from0.8 to 1.2, most preferably from 0.9 to 1.1.

In embodiments, the ratio [Ga] on [Zn] in the sputtering target may befrom 0.5 to 2.2, preferably from 0.7 to 2.2, more preferably from 0.8 to1.2, most preferably from 0.9 to 1.1.

In embodiments, the sputtering target may be polycrystalline.

The distance between the target and the substrate is not critical but adistance of from 50 to 100 nm is typically suitable.

It may be advantageous to cool the target during operation. Forinstance, the target may be water-cooled.

In step c, the film is formed on the substrate by sputtering the targetwith a sputtering gas comprising O₂.

Step c is typically performed under vacuum. For instance, a pressure offrom 0.05 to 5 Pa may be maintained during step c.

Step c is preferably performed by pulsed DC magnetron sputtering. Inembodiments, the pulsed DC reactive magnetron sputtering may beperformed with a frequency of from 10 kHz to 1 MHz, e.g. from 50 to 200kHz. Alternatively, AC magnetron sputtering can be used. In embodiments,the sputtering power may be at least 300 W, preferably at least 400 W,more preferably at least 500 W.

Either no duty on/off cycle or a duty on/off cycle can be used foroperating the DC reactive magnetron sputtering. If a duty on/off cycleis used, a duty on/off cycle of from 80/20 to 95/5 (e.g. 90/10) can beused.

In embodiments, the sputtering gas may be provided at a flow of from 10to 1000 sccm, preferably from 50 to 200 sccm, more preferably from 75 to150 sccm.

In embodiments, the sputtering gas flow may be composed for at least 35%oxygen, preferably at least 50% oxygen, yet more preferably at least 70%oxygen, yet more preferably at least 80% oxygen, even more preferably atleast 85% oxygen, yet more preferably at least 89% of oxygen. Thebalance, aside from oxygen, may be an inert gas or an inert gas andusual impurities. Typically, no impurities can be detected in thesputtering gas flow since the gas used can be obtained in very highpurity. The inert gas is preferably Ar.

In embodiments, in step c, the substrate may be at a temperature of from150 to 230° C., preferably from 175 to 220 ° C., yet more preferablyfrom 190 to 210° C.

In a second aspect, the present invention relates to a film of an oxideof In, Ga, and Zn, having a spinel crystalline phase, wherein In, Ga,and Zn represent together at least 95 at % of the elements other thanoxygen,

-   -   In represents from 0.6 to 44 at % of In, Ga, and Zn,    -   Ga represents from 22 to 66 at % of In, Ga, and Zn, and    -   Zn represents from 20 to 46 at % of In, Ga, and Zn,

In embodiments, the film of the second aspect may be obtainable by anyembodiment of the method of the first aspect.

Any feature of the second aspect may be as correspondingly described forthe first aspect. In particular, the formed oxide may be as described inthe first aspect.

In a third aspect, the present invention relates to an electronicdevice, such as a semiconductor device, comprising a film according tothe second aspect or an element, such as a channel, obtained frompatterning such a film. The semiconductor device may, for instance, be afield-effect transistor or a device comprising a field-effecttransistor, itself comprising said film according to the second aspector a channel obtained from patterning such a film. An example of such adevice comprising a field-effect transistor is a Dynamic Random-AccessMemory.

Any feature of the third aspect may be as correspondingly described forthe second or first aspect.

Particular and preferred aspects of the invention are set out in theaccompanying independent and dependent claims. Features from thedependent claims may be combined with features of the independent claimsand with features of other dependent claims as appropriate and notmerely as explicitly set out in the claims.

Although there has been constant improvement, change and evolution ofdevices in this field, the present concepts are believed to representsubstantial new and novel improvements, including departures from priorpractices, resulting in the provision of more efficient, stable andreliable devices of this nature.

The above and other characteristics, features and advantages of thepresent invention will become apparent from the following detaileddescription, taken in conjunction with the accompanying drawings, whichillustrate, by way of example, the principles of the invention. Thisdescription is given for the sake of example only, without limiting thescope of the invention. The reference figures quoted below refer to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are graphs of the XRD intensity (a.u.) as a function of20 (deg) for different substrate temperatures during sputtering with agas mixture O₂/Ar 80/20.

FIG. 2 is a graph plotting the maximal intensity (a.u.) observed inFIGS. 1(a) and (b) as a function of 2θ (deg).

FIG. 3 is a graph of the out-of-plane XRD intensity (a.u.) as a functionof 2θ (deg) for a substrate temperature of 200° C. during sputtering at500 W with different gas mixtures.

FIG. 4 is a graph of the XRD intensity (a.u.) as a function of 2θ (deg)for a substrate temperature of 200° C. during sputtering at differentpowers with a gas mixture O₂/Ar 80/20.

FIG. 5 is a graph of the grazing incidence XRD (1°) intensity (a.u.) asa function of 2θ (deg) for a substrate temperature of 200° C. duringsputtering with different gas mixtures.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims. The drawings described areonly schematic and are non-limiting. In the drawings, the size of someof the elements may be exaggerated and not drawn on scale forillustrative purposes. The dimensions and the relative dimensions do notcorrespond to actual reductions to practice of the invention.

Moreover, the terms top, bottom, over, under and the like in thedescription and the claims are used for descriptive purposes and notnecessarily for describing relative positions. It is to be understoodthat the terms so used are interchangeable under appropriatecircumstances and that the embodiments of the invention described hereinare capable of operation in other orientations than described orillustrated herein.

It is to be noticed that the term “comprising”, used in the claims,should not be interpreted as being restricted to the means listedthereafter; it does not exclude other elements or steps. It is thus tobe interpreted as specifying the presence of the stated features,integers, steps or components as referred to, but does not preclude thepresence or addition of one or more other features, integers, steps orcomponents, or groups thereof. The term “comprising” therefore coversthe situation where only the stated features are present and thesituation where these features and one or more other features arepresent. Thus, the scope of the expression “a device comprising means Aand B” should not be interpreted as being limited to devices consistingonly of components A and B. It means that with respect to the presentinvention, the only relevant components of the device are A and B.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment, but may. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner, as would beapparent to one of ordinary skill in the art from this disclosure, inone or more embodiments.

Similarly, it should be appreciated that in the description of exemplaryembodiments of the invention, various features of the invention aresometimes grouped together in a single embodiment, figure, ordescription thereof for the purpose of streamlining the disclosure andaiding in the understanding of one or more of the various inventiveaspects. This method of disclosure, however, is not to be interpreted asreflecting an intention that the claimed invention requires morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the claimsfollowing the detailed description are hereby expressly incorporatedinto this detailed description, with each claim standing on its own as aseparate embodiment of this invention.

All numbers including those expressing quantities of ingredients,reaction conditions, and so forth used in the specification are to beunderstood as being modified in all instances by the term ‘about.’Accordingly, unless indicated to the contrary, the numerical parametersset forth herein are approximations that may vary depending upon thedesired properties sought to be obtained. At the very least, and not asan attempt to limit the application of the doctrine of equivalents tothe scope of any claims in any application claiming priority to thepresent application, each numerical parameter should be construed inlight of the number of significant digits and ordinary roundingapproaches.

Where a range of values is provided, it is understood that the upper andlower limit, and each intervening value between the upper and lowerlimit of the range is encompassed within the embodiments.

All references cited herein are incorporated herein by reference intheir entirety. To the extent publications and patents or patentapplications incorporated by reference contradict the disclosurecontained in the specification, the specification is intended tosupersede and/or take precedence over any such contradictory material.

Furthermore, while some embodiments described herein include some butnot other features included in other embodiments, combinations offeatures of different embodiments are meant to be within the scope ofthe invention, and form different embodiments, as would be understood bythose in the art. For example, in the following claims, any of theclaimed embodiments can be used in any combination.

Furthermore, some of the embodiments are described herein as a method orcombination of elements of a method that can be implemented by aprocessor of a computer system or by other means of carrying out thefunction. Thus, a processor with the necessary instructions for carryingout such a method or element of a method forms a means for carrying outthe method or element of a method. Furthermore, an element describedherein of an apparatus embodiment is an example of a means for carryingout the function performed by the element for the purpose of carryingout the invention.

In the description provided herein, numerous specific details are setforth. However, it is understood that embodiments of the invention maybe practiced without these specific details. In other instances,well-known methods, structures, and techniques have not been shown indetail in order not to obscure an understanding of this description.

The invention will now be described by a detailed description of severalembodiments of the invention. It is clear that other embodiments of theinvention can be configured according to the knowledge of personsskilled in the art without departing from the technical teaching of theinvention, the invention being limited only by the terms of the appendedclaims.

Reference will be made to transistors. These are three-terminal deviceshaving a first main electrode such as a drain, a second main electrodesuch as a source and a control electrode such as a gate for controllingthe flow of electrical charges between the first and second mainelectrodes.

EXAMPLE 1 General Procedure for the Formation of a Film of an Oxide ofIn, Ga, and Zn

To deposit thin films of an oxide of In, Ga, and Zn, 300 mm diameter Siwafers were used as the substrate. The wafers had a 100 nm thick SiO₂film obtained by thermal oxidation to enable conductivity measurements.Similar results can, however, be obtained by deposition on the nativeoxidized Si surface. The presence of the insulating SiO₂ film isoptional when no conductivity measurements need to be performed.

Wafers were loaded via a vacuum load lock, entering the degas chamberfirst to remove adsorbed moisture by lamp heating at 350° C. in Arambient for 60 s. Next, wafers were placed in a physical vapourdeposition (PVD) chamber by robotic handling under vacuum. Inside thePVD chamber, the wafers were electrostatically clamped to a heated chuckby a bias voltage of 300 V. A backside Ar flow of 2-5 sccm throughchannels in the chuck enabled a good heat transfer. The wafertemperature was assumed to be same as the temperature measured andcontrolled by a thermocouple in the heated chuck. The temperature rangefor the present examples was 20-375° C. Before deposition, the chamberwas evacuated to a base pressure of 3×10⁻⁵ Torr by a cryogenic pump.

Film deposition was obtained by pulsed DC reactive magnetron sputteringwith the power of 200-500 W with a pulse frequency of 100 kHz and a dutyon/off cycle of 90%/10%. A 445 mm diameter IGZO disk was used as thetarget placed 78 mm above the wafer chuck. The IGZO disk consisted ofpoly-crystalline InGaZnO₄ (purity of 99.99%) with a metal atom ratio ofIn:Ga:Zn=1:1:1. The target was water-cooled to avoid excessive heating.The sputtering gas is a mixture of Ar and O₂. The total flow, includingthe back-side flow is kept at 100 sccm. At this flow, the pressure ismaintained at 0.5 Pa by continuous pumping via the cryogenic pump.

The obtained IGZO films have a metal composition close to that of thetarget. This did not depend on deposition temperature, power, or gasflow ratio. The morphology, however, was subject to change as will beshown in the examples below.

EXAMPLE 1.1 Formation of a Film of an Oxide of In, Ga, and Zn at VariousTemperatures

Example 1 was performed at various temperatures ranging from 20° C. to375° C. while setting the gas mixture at 20 at % Ar and 80 at % O₂ andusing a power of 500 W.

As can be seen in FIG. 1B, at room temperature, the film was amorphous.At 100° C., a spinel Fd3m (space group 227) phase appears. This iswitnessed by the appearance of peaks at Θ=35.5° in out-of-plane XRD (seeFIG. 3) and at 17.4° and 60° in grazing incidence XRD (GI-XRD, incidenceangle=1°, see FIG. 5) and are caused by respectively (222), (111), and(440) diffraction in the spinel structure. The peak at Θ=35.5° can beobserved in FIGS. 1A and 1B. The importance of the spinel phase growsfrom 100° C. to between 175° C. and 200° C. where it reaches a maximum.Then, it starts to decrease while a C-axis aligned crystalline (CAAC)phase appears and grows in importance. The appearance of CAAC IGZO inthe film can be detected by out-of-plane XRD, when a broad peak at Θ=30°appears. The intensity of this peak is a measure for the amount of CAACIGZO next to amorphous IGZO and increases with increasing depositiontemperature which has a stronger effect than changing the power orO₂-flow ratio. Above 250° C., the peak corresponding to the spinel phaseis not observed anymore.

EXAMPLE 1.2 Formation of a Film of an Oxide of In, Ga, and Zn in Pure Ar

Example 1 was performed at various temperatures ranging from 20° C. to375° C. while setting the gas mixture at 100 at % Ar and 0 at % O₂. Thepower was 500 W. The film was amorphous at all temperatures

EXAMPLE 1.3 Formation of a Film of an Oxide of In, Ga, and Zn in VariousOxygen Flow Ratio at 200° C.

Example 1 was performed at various O₂ flow ratios ranging from 5 to 95%with the balance being Ar. The temperature was set at 200° C. and thepower at 500 W. As can be seen in FIGS. 3 and 5, the peak at 0=35.5° isalready marginally present at an oxygen flow rate of 20% but reallystarts to emerge from the background between 30 and 40% O₂. It wasobserved that the higher the oxygen flow rate ratio, the more importantthe spinel phase becomes.

EXAMPLE 1.4 Formation of a Film of an Oxide of In, Ga, and Zn in VariousOxygen Flow Ratio at 300° C.

Example 1.3 was repeated at 300° C. Instead of obtaining a spinel phase,a CAAC phase was observed. Its importance grew with the oxygen flow rateratio.

EXAMPLE 1.5 Formation of a Film of an Oxide of In, Ga, and Zn in 80 at %Oxygen Flow Ratio Above 300° C.

Example 1 was performed at 80 at % O₂ flow ratio with the balance beingAr. The temperature was set at 350 or 375° C. and the power at 500 W. Atboth temperatures, the structure was polycrystalline. Both crystallinestructures were attributed to the hexagonal R3m (space group 166) unitcell.

EXAMPLE 1.6 Formation of a Film of an Oxide of In, Ga, and Zn atDifferent Powers

Example 1 was performed at an O₂ flow ratio of 80% with the balancebeing Ar. The temperature was set at 200° C. and the power was variedbetween 200 W and 500 W. As can be seen in FIG. 4, the spinel phase peakat Θ=35.5° is present at all powers but gains in intensity with higherpowers.

EXAMPLE 1.7 Formation of a Film of an Oxide of In, Ga, and Zn with aRatio In:Ga:Zn of 2:2:1

Example 1 is repeated while using a target having a ratio In:Ga:Zn of4:2:3 or 2:2:1. Preliminary results lead us to conclude that the targetshaving a ratio In:Ga:Zn of 2:2:1 or of 1:1:1 appear more prone to formthe spinel phase than the target having a ratio In:Ga:Zn of 4:2:3. Sofar, no target having a ratio In:Ga:Zn of 4:2:3 did form a spinel phase.

It is to be understood that although preferred embodiments, specificconstructions, and configurations, as well as materials, have beendiscussed herein for devices according to the present invention, variouschanges or modifications in form and detail may be made withoutdeparting from the scope of this invention. Steps may be added ordeleted to methods described within the scope of the present invention.

What is claimed is:
 1. A method for forming a film of an oxide of In,Ga, and Zn, having a spinel crystalline phase, the method comprising:providing a substrate in a chamber; providing a sputtering target in thechamber, the sputtering target comprising an oxide of In, Ga, and Zn,wherein: In, Ga, and Zn represent together at least 95 at % of elementsother than oxygen in the sputtering target, In represents from 0.6 to 44at % of In, Ga, and Zn in the sputtering target, Ga represents from 22to 66 at % of In, Ga, and Zn in the sputtering target, and Zn representsfrom 20 to 46 at % of In, Ga, and Zn in the sputtering target; andforming a film on the substrate, the substrate being at a temperature offrom 125° C. to 250° C., by sputtering the sputtering target with asputtering gas comprising O₂, the sputtering being performed at asputtering power of at least 200 W.
 2. The method of claim 1, wherein:In represents from 2 to 40 at % of In, Ga, and Zn in the sputteringtarget, Ga represents from 25 to 55 at % of In, Ga, and Zn in thesputtering target, and Zn represents from 23 to 43 at % of In, Ga, andZn in the sputtering target.
 3. The method of claim 2, wherein: Inrepresents from 10 to 40 at % of In, Ga, and Zn in the sputteringtarget, Ga represents from 30 to 50 at % of In, Ga, and Zn in thesputtering target, and Zn represents from 27 to 40 at % of In, Ga, andZn in the sputtering target.
 4. The method of claim 1, wherein the In,Ga, and Zn in the sputtering target represent together at least 99 at %of elements other than oxygen in the sputtering target.
 5. The method ofclaim 1, wherein the In, Ga, and Zn in the sputtering target representtogether at least 99.9 at % of elements other than oxygen in thesputtering target.
 6. The method of claim 1, wherein the In, Ga, and Znin the sputtering target represent together at least 99.95 at % ofelements other than oxygen in the sputtering target.
 7. The method ofclaim 1, wherein an atomic percent ratio of In to Ga in the sputteringtarget is from 0.51 to 1.96.
 8. The method of claim 1, wherein an atomicpercent ratio of In to Zn in the sputtering target is from 0.5 to 2.2.9. The method of claim 1, wherein an atomic percent ratio of Ga to Zn inthe sputtering target is from 0.5 to 2.2.
 10. The method of claim 1,wherein the sputtering is pulsed DC reactive magnetron sputtering. 11.The method of claim 10, wherein the pulsed DC reactive magnetronsputtering is performed with a frequency of from 10 kHz to 1 MHz. 12.The method of claim 1, wherein the sputtering power is at least 400 W.13. The method of claim 1, wherein the sputtering power is at least 500W.
 14. The method of claim 1, wherein a sputtering gas flow comprises atleast 80% of oxygen with a balance of the sputtering gas flow comprisingan inert gas.
 15. The method of claim 1, wherein a sputtering gas flowcomprises at least 85% of oxygen with a balance of the sputtering gasflow comprising an inert gas.
 16. The method of claim 1, wherein thesubstrate is at a temperature of from 150 to 230° C.
 17. The method ofclaim 1, wherein in step c the substrate is at a temperature of from 175to 220° C.
 18. The method of claim 1, wherein the spinel crystallinephase is of a space group Fd3m.
 19. A film of an oxide of In, Ga, andZn, having a spinel crystalline phase, wherein In, Ga, and Zn representtogether at least 95 at % of the elements other than oxygen, Inrepresents from 0.6 to 44 at % of In, Ga, and Zn, Ga represents from 22to 66 at % of In, Ga, and Zn, and Zn represents from 20 to 46 at % ofIn, Ga, and Zn,
 20. An electronic device comprising: the film of claim19, or an element obtained by patterning the film of claim 19.